Substances which are capable of absorbing large amounts of liquids are known in the art. Such substances have many uses; for example, such substances can be used in manufacturing products such as disposable paper pads having enhanced water absorbency. Such liquid-absorbing substances also find utility in the manufacture of products used to absorb bodily fluids, including such products as diapers, sanitary napkins, protective bedpads, and so forth.
Superabsorbent materials are a particularly preferred class of liquid-absorbing materials. A superabsorbent material, or "superabsorbent, " may be characterized as a material that absorbs at least about twenty times its own weight of an aqueous liquid, such as water or bodily fluid. Superabsorbents do not dissolve in the liquid, and tend to retain their general shape regardless of the amount of liquid absorbed. Upon absorbent contact with a liquid, a superabsorbent will swell to its equilibrium volume, and will have sufficient physical integrity to resist flow and fusion with neighboring superabsorbent particles or structures. When liquid is absorbed by the superabsorbent, a gelatinoid product will be formed.
The prior art has provided a number of superabsorbent materials having excellent properties for use in a variety of applications. For example, as taught in U.S. Pat. No. 3,661,815, issued to Theodore Smith and assigned to Grain Processing Corporation, Muscatine, Iowa, the prior art has provided alkali methylcarboxylate salts of starch-acrylonitrile graft copolymers that are capable of absorbing water in amounts in excess of fifty parts per part thereof. Such polymers may be prepared by the graft copolymerization of an acrylonitrile polymer with starch, as taught, for example, in U.S. Pat. No. 4,069,177, issued to Theodore Smith of Muscatine, Iowa. A liquid-absorbent material may be prepared from such polymer by saponification of the resulting starch-acrylonitrile graft copolymer with a base in an aqueous alcoholic medium. The liquid-absorbent material thus prepared will be satisfactory for use in a variety of liquid-absorbent applications. Such superabsorbent materials are sold by Grain Processing Corporation as the WATER LOCK.TM. family of superabsorbents. These materials can absorb liquids in an amount of up to several hundred times their own weight.
As further taught in U.S. Pat. No. 4,558,100, issued to Adrian P. Kightlinger, et al., and assigned to Grain Processing Corporation, an improved absorbent composition may be prepared by treating an aqueous mixture of a polynitrile species, such as acrylonitrile or methacrylonitrile, and a poly-functional monomeric cross-linking agent with a polymerization initiator to achieve polymerization and cross-linking of the starch-polynitrile polymer. The resultant cross-linked polymer is then saponified, using an aqueous alcohol solution of an alkali metal base. Further improvements may be realized by using an oxidizing agent, a reducing agent, or a combination of an oxidizing agent and a reducing agent in the manufacture of the starch-polynitrile graft copolymer, as taught in U.S. Pat. No. 4,769,414, issued to Adrian P. Kightlinger et al. and assigned to Grain Processing Corporation.
The prior patents discussed herein provide excellent teachings as to absorbent compositions and methods for their manufacture. The absorbent compositions prepared in accordance with the foregoing teachings may advantageously be used in a variety of superabsorbent applications, such as diapers, protective bed coverings, and so forth. One minor drawback of the prior art compositions, not affecting the utility of these compositions in such applications as described hereinabove, is that, upon absorbing of an aqueous composition, the gelatinoid product thus formed lacks substantial clarity, but rather possesses an occluded or cloudy appearance. While such appearance is of no importance in conventional superabsorbent applications such as diapers and the like, the lack of clarity of the gelatinoid product potentially limits other applications to which the superabsorbent material may be put.
It is a general object of the invention to provide a liquid-absorbent material that forms a substantially clear gelatinoid product, i.e., a gelled or gel-like product, when the liquid-absorbent material is mixed with an aqueous liquid in an amount sufficient to provide such gelatinoid product. It is another general object of the invention to provide a substantially clear gelatinoid product, and further to provide methods for preparing the liquid-absorbent material and gelatinoid product.